Metabolic processes have long been proposed for anabolic and catabolic bioconversions. Microorganisms of various types have been proposed for these bioconversions and include bacteria and archaea, both of which are prokaryotes; fungi; and algae. Metabolic processes are used by nature, and some have been adapted to use by man for millennia for anabolic and catabolic bioconversions ranging from culturing yogurt and fermentation of sugars to produce alcohol to treatment of water to remove contaminants. Metabolic processes offer the potential for low energy consumption, high efficiency bioconversions in relatively inexpensive processing equipment and thus may be and are often viable alternatives to chemical synthesis and degradation methods. Often anabolic processes can use raw materials that are preferred from a renewable or environmental standpoint but are not desirable for chemical synthesis, e.g., the conversion of carbon dioxide to biofuels and other bioproducts. Catabolic bioconversions can degrade substrates and have long been used for waste water treatment. Considerable interests exist in improving metabolic processes for industrial use and expanding the variety of metabolic process alternatives to chemical syntheses and degradations.
In some instances difficulties can occur where it is not desirable to contact the gaseous or liquid feedstock containing the substrate with the aqueous medium containing the microorganism for the metabolic process. For example, the gaseous or liquid feedstock may contain components that could build up in the aqueous medium such as solids; or a liquid feedstock containing the substrate may result in dilution of the aqueous medium requiring large reactor sizes. Additionally, the feedstock may not be aqueous or may contain two liquid phases. Introducing a gas phase feedstock into an aqueous medium may require compression of the gaseous medium in order to overcome the hydraulic head of the aqueous medium resulting in capital and operating expense.
Especially for substrates that are sparingly soluble in water, economic viability of commercial-scale bioconversion processes will not only depend upon the bioconversion rate and efficiency but also on the rate of mass transfer of the substrate from the gas phase to the aqueous phase. The mass transfer rate will be reflected, in part, by the surface area between the gas and liquid phases and the duration of contact. Accordingly, proposed bioreactors trend toward the use of smaller bubbles of gas and contact times sufficient to enable a desired amount of mass transfer of gas into the aqueous phase to be achieved both of which can add to capital and operating expenses. The challenges faced are even greater where the substrate is in a low concentration in the gaseous fluid.
Bioreactor designs have been proposed for treating gas phase feedstocks where the microorganisms are contained on a solid structure. Typically these reactors maintain the microorganisms and the solid structure externally wet in order to maintain the microorganism.
Birdwell, et al., in U.S. Pat. No. 5,409,823 disclose an apparatus for removing pollutants from air by spraying a microbial laden liquid into incoming polluted air in a wet plenum chamber having a liquid level therein. The air passing out of the wet plenum chamber enters a wet sill chamber having a filtration medium that is sprayed with liquid laden microbial agents to provide further dwell time. The patentees suggest that their process is useful for the removal of volatile organic compounds, air toxins and odors.
Apel in U.S. Pat. No. 5,795,751 discloses a biofilter for the removal of nitrogen oxides from contaminated gases under aerobic conditions. The biofilter is a porous, organic filter bed, preferably wood compost. At column 3,lines 33 et seq., the patentees state that moisture lost can be replenished periodically by the addition of a liquid, such as a buffer solution, to the compost.
Barshter, et al., in U.S. Pat. No. 5,821,114, disclose a biofilter using modular panels where contact between the gas and the microbial population on the filter removes contaminants. The patentees propose the use of the filter for the removal of hydrocarbons, reduced sulfurs, ammonia, and the like. At column 3, lines 57, et seq., the patentees state that moisture is preferably added periodically by means of sprinklers or perforated hose.
Breckenridge in U.S. Pat. No. 6,117,672 discloses a moving bed biofilter and condenser for flue gas pollutant removal and collection. In his process, a moving belt conveying a wet mat of chopped biomass impregnated with bacteria that feed on nitrogen oxides is used. The gas is passed through the belt.
Ren, et al., in U.S. Published Patent Application No. 2012/0208262 disclose improved biotrickling filters for treating waste gas. Waste gases pass through a packed bed which is maintained wet using sprays. The apparatus involves changing the direction of the gas flow.
Another trickle bed is discussed by Jiang, et al., in Nitrogen oxide removal from flue gas with a biotrickling filter using Pseudomonas putida, Journal of Hazardous Materials, 164, pages 432-441 (2009). The authors noted several practical problems with trickle bed bioreactors. First, the pressure drop through the bed can be material in commercial units where using the smaller diameter supports (about 2 to 3 millimeters in diameter) to provide high surface area per unit volume. Second, microbial contamination can occur. Third, biofilm build up can occur that can cause failure of the system. Fourth, the microorganisms, and sought biofilms, can be washed from the surface of the supports thereby making backwashing difficult. And fifth, start-up of bioreactors to enhance adhesion of the microorganisms on the support may require a laborious empirical approach.
A yet another approach is to use biofilm membranes where the gas to be treated is maintained on one side of the membrane and an aqueous medium is provided on the other side. Microorganisms may form a biofilm on aqueous medium side of the membrane. A driving force provides for the permeation of the sought substrate through the membrane where bioconversion occurs. Energy is required for the transport of the substrate through the membrane, which has to have sufficient strength to provide physical integrity. Moreover, the formation of excess biofilms or microbial contamination can adversely affect the performance of the biofilm membranes.
Accordingly improved processes are sought for bioconverting substrate where the feedstock supplying the substrate presents challenges to bioconversions where the microorganisms or enzymes need to be retained in an aqueous medium.